Method of vacuum casting metal

ABSTRACT

A metal casting system especially adapted for the rapid melting and casting of small charges of metal, utilizing resistance heating of such charge and immediate delivery of the molten metal to a suction mold.

United States Patent Daniel et a1. 1 1 Jan. 23, 1973 54 METHOD OF VACUUMCASTING 2,024,615 12/1935 Touceda ..164/4 METAL 2,685,718 8/1954Schmitz... ....164/255 0 2,997,756 8/1961 Strom ..l64/62 [761 lnvenmrsfl9 Dame, 1 DeVmSh'm 3,434,527 3/1969 Bedell ..164/65 x Drive; J p Rlchey,1 2160 3,435,878 4 1969 Howard 6191.. ..164/66X Demingwn Drive, both ofCleveland 3,494,199 2/1970 Stacey .164/4 x Heights, Ohio 44106 FOREIGNPATENTS OR APPLICATIONS [22] Filed: Nov. 25, 1970 991.240 1 5 1965 G B64 80 211 App1.No.: 92,583 I 1 l Primary ExaminerR. Spencer Annear [52]11.8. C1. ..l64/4, 164/63, 164/66, A rney-Oberlin, Maky, Donnelly &Renner 164/255 [51] Int. Cl. ..B22d 27/16 [57] ABSTRACT [58] Field ofSearch iii/ 55 3 2 A metal casting ystem especially adapted for therapid melting and casting of small charges of metal, utilizingresistance heating of such charge and im- [56] References Cl-ted mediatedelivery of the molten metal to a suction UNITED STATES PATENTS mold-1,659,638 2/1928 Shapiro ..164/255 X 10 Claims, 5 Drawing Figures 1PATENTEUJAH 23 ms 3,712,364

sum 3 or 3 INVENTORS. ARNOLD DAN/EL BX JOSEPH B. R/C'HEY, 1T 3%), uamnblflmnF Z ATTORNEYS.

METHOD OF VACUUM CASTING METAL BACKGROUND OF THE INVENTION Investmentcasting procedures have long been em ployed in the production ofinvestment castings of precious metals for use in jewelry, dental inlaysand bridges, and the like. The present system is particularlyadvantageous when very precise control of the casting operation isrequired in order that the casting may be of exact predetermineddimensions and alloy composition.

In the conventional production of dental alloy castings, an investmentmold in prepared about a wax model or pattern and the mold is preparedabout a wax model or pattern and the mold is preheated to volatilize thewax and to bring the mold up to the desired casting temperature. Theachievement and maintenance of such temperature during the castingoperation is of considerable importance inasmuch as both the mold andthe casting tend to shrink upon cooling and in order to obtain afinished casting of exact predetermined dimensions it is essential thatboth the molten metal and the mold be at proper temperatures when themolten metal is introduced into such mold. In a typical operation, themold has thus been preheated and then transferred to a centrifugalcasting machine. The gold alloy charge is placed in the machine andheated by means of an oxyacetylene torch to melt the charge, whereuponthe machine is activated and the molten metal forced into the mold underthe influence of centrifugal force. Under such circumstances, it isobviously not possible to ensure that the mold is at the precise optimumtemperature at the moment the molten metal enters the same and it isalso not possible to ensure that the molten charge itself is at anoptimum temperature. Furthermore, the alloy composition may be somewhatmodified during the heating operation and the charge may be contaminatedby atmospheric gases and the gases produced by employment of such torch.An appreciable excess of the alloy must be provided requiring subsequentcutting and grinding away of the metal sprue produced, and the entireoperation requires such a degree of skill and experience that manydental offices prefer to send such work to specialist laboratoriesrather than to perform the work themselves. This results in considerabledelay and increased expense, particularly when several castings must beproduced until one is achieved which is found to be entirelysatisfactory for the intended purpose.

It is accordingly an object of the present invention to provide aself-contained unit wherein the mold need not be removed from the heatbath prior to or during performance of the casting operation.

It is also an object to provide a metal casting system and apparatus forperforming the same of small size which may be operated from theordinary l 10 volt Ac source available in all dental offices and whichwill incorporate a control system to ensure the automatic production ofhigh quality castings without the requirement of any special skills onthe part of the operator.

A further object is to provide a metal casting system wherein both themolten metal and the mold are brought to the optimum temperatures andthe casting operation is performed very rapidly in a manner to keep themolten metal charge uncontaminated and to maintain the proportions ofany alloy elements unchanged.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described, the followingdescription and the annexed drawings setting forth in detail certainillustrative embodiments of the invention, these being indicative,however, of but a few of the various ways in which the principles of theinvention may be employed.

DESCRIPTION OF THE DRAWING In said annexed drawing:

FIG. 1 is an isometric view of a novel metal casting apparatus,including associated control means, for practicing the method of thepresent invention;

FIG. 2 is an enlarged vertical longitudinal section taken on the line2-2 of FIG. 2, but omitting the control panel;

FIG. 3 is a wiring diagram illustrating suitable control means for theoperation of such apparatus;

FIG. 4 is an enlarged exploded isometric detail view of the meltingcrucible assembly of such apparatus; and

FIG. 5 is a wiring diagram of an optional modification of the metalheating means.

DESCRIPTION OF THE INVENTION In accordance with the invention,investment casting apparatus is provided comprising an insulatedenclosure or muffle having heating means for heating the interiorthereof to a high temperature. A metal heating crucible is mounted inthe upper portion of such enclosure and means is provided to support amold directly beneath such crucible in clamping engagement therewith sothat molten metal produced in the crucible may be directly dischargedinto the mold therebeneath. Suction means is applied to the mold toassist in drawing the molten metal charge from the crucible into themold cavity, and electrical control means ensure that the variousoperations are carried out at the proper times and at the propertemperatures. The melting crucible may be enclosed within an inertatmosphere to avoid any possibility of contamination of the moltencharge.

Now referring more particularly to said drawing and especially FIGS. 1and 2 thereof, the embodiment of the invention there illustratedcomprises a furnace or oven 1 desirably supported on legs such as 2 andhaving an outer sheet aluminum shell or casing 3. Such casing is linedwith a thick wall of refractory insulating material 4 which encloses arefractory ceramic inner liner 5. Nichrome heating elements 6 areinterposed between such outer liner 4 and inner liner 5 in order toraise the interior of the furnace to a high temperature. The front ofthe furnace is closed by an insulated hinged hinged gate or door 7normally held in closed position by means of counterweight 8.

A control panel 9 is carried by brackets spacing the same from the endof the furnace, and a removable hood 10 is mounted on the top of thefurnace. Such hood may be connected, if desired, to a source of inertgas such as cylinder 11 by means of flexible tubing 12, admission ofsuch gas (e.g. helium, nitrogen, or argon) to the hood being controlledby means of hand operated valve 13.

Hood 10 is adapted to cover an opening in the upper side of the furnacewithin which a metal heating crucible is received and mounted. Suchcrucible is a vertically split graphite crucible comprising halfportions 14 and 15 (see also FIG. 4) separated by a layer of inertrefractory material such as asbestos 16 which may be provided with acentrally disposed gap in the lower edge thereof to serve as a sprue forthe crucible; the two halves of the crucible are thus electricallyinsulated from each other. In the preferred embodiment illustrated indetail in FIG. 4 the sprue 17 is, however, provided in one crucible halfonly, with the inner surfaces of the other half and such asbestosdivider being inclined theretoward. Such sprue may, for example, beapproximately 0.04 mm. diameter. Accordingly, when the crucible blocks14 and 15 are assembled with the interposed insulating layer 16 theydefine a crucible cavity the upper end of which projects above the uppersurface of furnace l and the lower end of which projects into theinterior of furnace l. The graphite blocks 14 and 15 are respectivelybacked by heavy sheet aluminum or copper contact plates 19 and 20 to theupper edges of which electric power cables 21 and 22 are connected.Layers of electrical insulation 23 and 24 laterally enclose the crucibleand contact plates so that the two halves of the crucible are not onlyelectrically isolated from each other but also from the supporting upperfurnace wall.

An optional, but ordinarily highly preferred device is also providedwithin hood 10 to bear downwardly upon a metal charge to be meltedwithin the crucible and thereby ensure better contact between the twohalf sections of such crucible and the charge which bridgestherebetween. Such device may comprise a bracket 25 mounted on the uppersurface of the furnace and carrying a vertically disposed plunger 26directly axially reciprocable above the sprue of the graphite meltingcrucible. A compression spring 27 serves normally to maintain suchplunger in depressed condition so that the graphite rod 28 coaxiallysecured thereto may thus bear firmly downwardly upon the charge in thecrucible. Plunger 26 and rod 28 may be temporarily manually elevatedwhen it is desired to place a new charge within the crucible bydepressing lever handle 29.

A circular metal, e.g. stainless steel, jig or platform 30 is supportedwithin the furnace chamber or muffle 31 on the upper end of verticallyreciprocable metal tube 32. The upper end of such tube opens to acentrally disposed orifice in the upper surface of platform 30 and thelower end of such tube connects through an elbow 33 of stainless steeltubing to vacuum hose 34. A

manually rotatable nut 35 is threaded on the lower projecting endportion of metal tube 32 and held against axial movement by upper washer36 interposed between such nut and the underside of the furnace and bylower washer 37 interposed between such nut and the centrally aperturedspider 38 affixed to the underside of the furnace and projectingdownwardly therefrom. Accordingly, when nut 35 is thus manually rotated,jig or platform 30 'will be correspondingly elevated or lowered withinthe furnace chamber.

The remaining elements of the assembly may best be understood from thedescription of the operation of the metal casting system set forthbelow.

OPERATION The operation of this new metal casting system will first begenerally described, and then more specifically with reference to theelectric control system.

The usual wax pattern or model is prepared and embedded in a suitableporous plaster mold 39 enclosed in the usual tubular metal casting ring40. Such mold may desirably initially be placed within furnace chamber31 laterally of platform 30 as shown at 40' for preheating prior toperformance of the casting operation. Such preheating serves tovolatilize and drive off the wax pattern, the fumes being vented fromchamber 31 through vents V, and to heat the mold 39 to proper castingtemperature. The door to the furnace will then be opened and theoperator will lift the mold onto platform 30 where it is supported onannular asbestos sealing ring 41 which serves thus to space theunderside of the mold slightly from the upper surface of platform 30,thereby permitting suction subsequently to be applied to the entireundersurface of the mold through metal tube 32. In some cases, if themold is not sufficiently porous, it may be desirable to provide a verysmall vent passage 42 leading from the mold cavity 43 to a layer ofporous asbestos 44 extending to the lower surface of mold 39 and therebypermitting suction (e.g. 28-29 inches of mercury) to be applied moredirectly to such mold cavity. The upper surface of the mold is crateredat 45 to provide a funnel shaped cavity terminating in a small sprue 46leading to the mold cavity 43. A centrally apertured sheet metal cover47 fits over the top of the mold and asbestos sealing ring 48, and asmaller asbestos sealing ring 49 is interposed between such cover andthe underside of the crucible so that the sprue 17 of the cruciblecommunicates directly with a closed vacuum system.

A crucible assembly is clamped firmly in place by means of large setscrew 50.(which may be insulated from contact plate 19, or. which maymerely be grounded by grounding of outer aluminum casing 3 of thefurnace). The hand nut 35 will now be rotated to elevate platform 30 toraise the mold and clamp cover 47 firmly against sealing ring 49interposed between the'same and the underside of the split crucible.

Cover 10 is now lifted, handle 29 is depressed to raise rod 28, and themeasured solid metal charge 51 is placed in the crucible. Handle 29 isthereupon released so that rod 28 may firmly press such chargedownwardly against the respective crucible half sections. Heatingcurrent may now be directed to the crucible sections through leads 21,22 for a sufficient period of time (e.g. l0 seconds) to melt the metalcharge. The plunger assembly 26, 27, 28 will ordinarily be sodimensioned and proportioned that, even when the crucible is completelyempty, the lower end of rod 28 will not extend entirely to the bottom ofthe crucible, it being desired only that the lower end of such rodshould bear upon the solid metal charge with sufficient force to ensureadequate electrical contact between such charge and the respectivecrucible sections. When the charge has melted and been brought tooptimum temperature (as determined by thermocouple or infrared detectorsensing means described in more detail in connection with the discussionof the wiring diagram, below) suction is applied to tubing 34 andtherefore through elbow 33, tubular support 32, porous mold 39, moldcavity 43, and sprue 46 to the crucible sprue to overcome the surfacetension of the molten metal and rapidly draw the latter downwardly intothe mold cavity. Platform 30 may thereupon be lowered, door 7 opened andthe mold removed from chamber 31. Alternatively, if it is desired tocool the mold more slowly in order better to control the solidificationphenomena which affect the grain structure of the casting, the mold maybe left within such chamber for an extended period of time while thechamber is very gradually cooled.

It will be apparent from the foregoing that the molten metal may thus bevery rapidly drawn into the mold cavity immediately upon attainment ofoptimum temperature and with minimal opportunity to absorb anyimpurities. As above explained, an inert gas may be admitted to cover orhood 10 from cylinder 11 prior to performance of the melting operationand, if desired, such inert gas may even be admitted under sufficientpressure substantially to purge the hood and crucible prior to elevationof the mold into clamping relationship with the underside of thecrucible. The inert gas pressure within the hood may thereupon assist inejecting the molten metal from the crucible into the evacuated moldcavity if the hood is tightly clamped down and the crucible isadequately sealed at the top of the furnace.

Now referring more particularly to FIG. 3 of the drawing, the operationof the metal casting system of this invention will be described ingreater detail, with particular reference to the electrical controlsystem. After the investment rings 40 and 40 with their enclosed moldshave been placed within furnace chamber 31, and door 7 closed, lineswitch 52 will be manually closed to admit 1 10 volt AC heating currentfrom main line 53, 54 to lines 55, 56 connected with the nichromeheating elements 6 in furnace 1. Line 56 is connected through a standardtemperature control unit 57 which may be preset to disconnect such linewhen a desired temperature has been attained within the furnace.Temperature sensing means in the form of the thermocouple 58 may beprovided in the furnace and connected with such temperature control unit57 to activate the latter. A neon pilot light 59 signals that power ison.

Hand screw 35 is rotated to elevate platform 30 to clamp the mold inplace beneath the split crucible, and the metal charge 51 is placedwithin such crucible and downwardly biased rod 28 is caused to pressthereon. Hood 10 is closed, clamped, and inert gas is admitted theretoif desired.

When thermocouple 58 signals that the predetermined temperature has beenattained in furnace chamber 31, the temperature control unit 57automatically cuts off or limits further power to the nichrome heatingelements 6 to maintain the constant preset temperature. After sufficienttime is allowed for the mold to reach optimum temperature, the operatormay thereupon initiate the melting cycle by depressing the reset switch60 on crucible temperature controller 61. This energizes the coil ofcontactor 62 which in turn applies line voltage to the primary oftransformer 63, causing very high current to flow through the secondaryof such transformer, the two graphite mold sections 14 and 15, and themetal charge 51 bridging the latter.

When the predetermined melting optimum temperature of the molten metalproduced has been sensed and signaled by thermocouple 64 embedded in thecrucible but electrically insulated therefrom or by infrared detector Dlocated above the crucible, the temperature control unit 61 deenergizesthe coil of contactor 62 and energizes the coil of time delay relay 65which in turn energizes the coil of normally closed solenoid valve 66 toopen the latter and connect suction line 34 to vacuum for a length oftime determined by the time delay relay. This immediately operates asabove described to draw the molten metal through the crucible sprue intothe evacuated mold cavity 43. After a preset time determined bytime-delay relay 65, valve 66 automatically recloses, cutting off thevacuum connection to the mold.

During application of the vacuum, the pressure differential is effectiveto overcome the surface tension of the molten metal and force the latterthrough the small crucible sprue.

It will be seen from the foregoing that a method of discharging moltenmetal from the crucible has been provided wherein the outlet or sprueleading therefrom is of sufficiently small size that the surface tensionof the molten metal normally serves to prevent discharge therethrough bygravity, the suction which is then applied at the proper momentaffording a pressure differential sufficient to overcome such effect ofsurface tension and cause the molten metal to flow through the sprue.The crucible should, of course, be of material not wettable by suchmolten metal.

The investment may be prepared in generally wellknown manner for theproduction of cast dental inlays, onlays, crowns, bridges, andframeworks for removable appliances, utilizing dental casting goldalloys in accordance with American Dental Association specification No.5 (Federation Dentaire Internationale Specification No. 7 for DentalCasting Gold Alloy) Approved Apr., 1965; Effective Apr. 1, 1966.

The wax pattern is prepared and a 12 gauge (Brown & Sharpe) sprue pin isinserted in the bulkiest part of the pattern and a conical sprue formeris affixed to the other end of such pin. Normally, the extent of suchpin from the pattern to the sprue former should not exceed three-eightsinch. The assembly is carefully washed and allowed to dry.

A cylindrical brass or stainless steel casting ring is lined with acircularized strip of asbestos, the ends of which overlap slightly. Suchring may be 1 h inch in length and 1 A inch in diameter, for example,and the asbestos liner should be about one-eighth inch shorter than thering, at each end.

The investment plaster (usually a gypsum composition) such as KerrLuster Cast or Cristobolite is mixed with water (e.g. 30 to 34 parts ofwater to grams of powder), the plaster being sifted into the water,until a creamy consistency is obtained. The pattern is painted with themixture, usually after first lightly painting it with a wetting agent,and inverted and placed within the ring which is then filled with thesame mix.

After being allowed to set for about 1 hour, the burnout operation maybe commenced. Thesprue'formei" and sprue pin are removed and the castingring and investment may now be placed in the furnace with the sprue holein the down position. When the mold has gradually been brought toapproximately 500 F it is turned with the sprue upward and brought tocasting temperature (l292 F in reference to Cristobolite). It shoulddesirably be allowed to heat soak for about 20 minutes at thistemperature prior to casting.

The crucible is charged with ingots of gold alloy (about 2 to 4pennyweight for inlays and crowns, and about 20 to 30 pennyweight forlarge partial framework castings) and brought to fusion temperature,this ordinarily being nearer the upper (liquidus) than the lower(solidus) limit of the melting range.

The mold is clamped and the vacuum applied as described above. Thevacuum should be high, e.g. 28-29 inches of mercury, if possible, (l2-l3psi) and the diameter of the crucible sprue opening may be 16 gauge(Brown & Sharpe) or less to prevent premature dribbling.

Now referring more specifically to FIG. 5 of the drawing, the optionalform of hold-down there illustrated may comprise a graphite electrode 67corresponding to rod 28 in function but also being electricallyconnected to transformer 63. It must accordingly be electricallyinsulated as at 68 in its support, and a portion of the heating currentwill flow between the electrode and the crucible sections.

The temperature sensing thermocouple 64 (FIGS. 3 and 4) may have itsleads extending through an elongated ceramic insulating member 69inserted in one of thegraphite crucible blocks with the thermocoupleitself embedded in refractory ceramic insulating cement adjacent themetal fusing zone. Insulating pads 70 and 71 may be provided on thecrucible projections such as 72 and 73 to ensure that the crucible isinsulated from the furnace under the upward pressure of threaded tube 32and also to provide a gas tight seal around the crucible.

The infrared radiation detector D referred to above as a preferredalternative to thermocouple 64 is a commercially available unitsemi-diagrammatically shown in FIG. 2 as mounted in an extension ofbracket 25.

- Such detector (preferably the silicon type) is clamped at an angle tofocus upon the charge 51 in the bottom of the crucible and thereby isenabled to sense the temperature of such charge by response to theinfrared radiation of the metal (infrared radiation pyrometry). Theleads from detector D thus connect with temperature controller 61instead of the leads shown coming from thermocouple 64 in FIG. 3.

The apparatus and method of the present invention accordingly provide ametal casting system adapted to ensure the integrity of the resultingcasting as to com position, physicals, and dimensions to a degree notheretofore readily attainable. Such apparatus is not unduly expensive,and the mode of operation is such that it may be carried out without thebenefit of exceptional skill and training.

We therefore particularly point out and distinctly claim as ourinvention:

1. A method of casting which comprises preheating a mold to optimumcasting temperature, and while maintaining such mold at such optimumcasting temperature, passing electric current through a metal charge ina container above such mold to head rapidly such charge to melt the sameand bring the molten metal to optjmum casting temperature within suchcontainer w rch has an opening ll'l communication with the moldtherebeneath that is too small to permit ready passage of such moltenmetal therethrough, and then while maintaining such molten metal atoptimum casting temperature subjecting such molten metal to sufficientpressure differential to force such molten metal through such openinginto such mold.

2. The method of claim I wherein such mold is a suction mold, andsuction is applied to such mold to provide the pressure differential todraw such molten metal from such container into such mold.

3. The method of claim 1 wherein the surface tension of such moltenmetal is sufficient to inhibit passage of the metal through the openingof such container under the influence of gravity, and such pressuredifferential is effective to overcome such surface tension when applied.

4. The method of claim 1 wherein the temperature of the metal charge issensed during heating thereof and such pressure differential is abruptlyapplied to force the molten metal through the container opening into themold after the molten metal has reached the optimum casting temperature.

5. The method of claim 4 wherein the temperature of the metal charge iselectrically sensed.

6. Themethod of claim 4 wherein an infrared radiation detector isfocused upon the charge ,in the container to sense the temperature ofsuch charge by response to the infrared radiation of the metal.

7. The method of claim 1 wherein the container and mold are brought intotight sealing engagement with each other prior to melting the metalcharge, and suction is applied to the mold to provide the pressuredifferential on the molten metal in the container which draws suchmolten metal into the mold.

8. The method of claim 1 wherein such melting operation is carried outin an inert atmosphere.

9. The method of claim 1 wherein such container comprises twoelectrically conductive halves which are insulated from each other, andpressure is applied to the metal charge within the container to insureadequate electrical contact between the metal charge and such containerhalves for passage of electric current through the metal charge whensuch electric current is supplied to the container halves.

10. The method of claim I wherein such optimum casting temperature isnearthe upper liquidous limit of the metal charge.

2. The method of claim 1 wherein such mold is a suction mold, andsuction is applied to such mold to provide the pressure differential todraw such molten metal from such container into such mold.
 3. The methodof claim 1 wherein the surface tension of such molten metal issufficient to inhibit passage of the metal through the opening of suchcontainer under the influence of gravity, and such pressure differentialis effective to overcome such surface tension when applied.
 4. Themethod of claim 1 wherein the temperature of the metal charge is sensedduring heating thereof and such pressure differential is abruptlyapplied to force the molten metal through the container opening into themold after the molten metal has reached the optimum casting temperature.5. The method of claim 4 wherein the temperature of the metal charge iselectrically sensed.
 6. The method of claim 4 wherein an infraredradiation detector is focused upon the charge in the container to sensethe temperature of such charge by response to the infrared radiation ofthe metal.
 7. The method of claim 1 wherein the container and mold arebrought into tight sealing engagement with each other prior to meltingthe metal charge, and suction is applied to the mold to provide thepressure differential on the molten metal in the container which drawssuch molten metal into the mold.
 8. The method of claim 1 wherein suchmelting operation is carried out in an inert atmosphere.
 9. The methodof claim 1 wherein such container comprises two electrically conductivehalves which are insulated from each other, and pressure is applied tothe metal charge within the container to insure adequate electricalcontact between the metal charge and such container halves for passageof electric current through the metal charge when such electric currentis supplied to the container halves.
 10. The method of claim 1 whereinsuch optimum casting temperature is near the upper liquidous limit ofthe metal charge.